Balloon catheters are used in surgical techniques, such as angioplasty, in which constrictions in the vascular system (usually coronary arteries) are removed by placing the balloon of the catheter at the site of the constriction and inflating the balloon by applying a gas or fluid to the ballon through the bore of the tubular portion of the catheter to which the balloon portion is attached, typically to a pressure of the order of 5 to 20 bar. This expands the blood vessel radially locally at the balloon to remove the constriction. This technique is well established, but suffers from the disadvantage that 40% of expanded constrictions spontaneously collapse within 24 months of insertion of the balloon. In order to prevent such spontaneous collapse, a rigid tubular reinforcing lining (known as a stent) is commonly placed at the constriction site and expanded radially into position by the balloon catheter so as to provide a more permanent support for the radial expansion of the blood vessel.
Conventional balloon catheters typically comprise a tubular portion carrying the balloon portion at or adjacent the distal end of the tubular portion. The proximal end of the tubular portion is connected to a source of gas or liquid under pressure which is used to expand the balloon portion radially when it has been located at the correct position within a blood vessel. The balloon catheters are of two main types:                those in which the balloon portion is initially of a narrow radial diameter and is expanded radially by the application of pressure to form a larger diameter ballon portion by stretching the wall of the balloon portion and are known as compliant catheters; and        those which have a balloon portion, usually made from a thin walled polyethylene terephthalate (PET), which has the required final radial dimension and which is inflated without causing significant radial stretching of the balloon and are known as non-compliant catheters.        
In the compliant catheter, that portion of the tube which is to form the balloon portion of the catheter is made from an elastic polymer, so that it can stretch radially to form the larger diameter balloon portion. Usually, such a catheter incorporates reinforcing polymer or metal fibres or braided fibres which not only provide mechanical support to the wall material of the balloon, but also restrict the extent to which the ballon can expand radially. The braiding allows a range of elastic polymers to be used for the wall material and enables high inflation pressures to be used. Typically, such a catheter is formed by laying up the various plies of the structure on a former and removing the former axially to produce a tubular member having a multi-ply wall of substantially uniform thickness. Examples of such compliant catheters are those described in PCT Application No WO 87/00442 and European Patent Application No 0 425 696 A1. However, as described in WO 87/00442, problems arise with such compliant catheters in that the balloon portion moves axially within the blood vessel as the balloon portion is inflated. In order to overcome this, as described in the PCT Application complex design of the relative angles between the fibres in the braiding are required to ensure that as the balloon portion expands other portions of the catheter tube expand axially to retain the balloon portion in the same axial position within the blood vessel. Such forms of catheter are complex and expensive to manufacture and require that the various plies of the structure of the balloon portion are free to move relative to one another to accommodate the changes in geometry of the wall shape as the balloon inflates. Furthermore, as the balloon portion is expanded radially within the blood vessel, the wall thickness reduces, weakening the balloon portion.
With the non-compliant type of catheter balloon, the balloon is made from a substantially non elastic polymer, notably a PET, so that the balloon will expand radially only to its fully deployed state. Such catheters are typically made by blow moulding the desired balloon portion and affixing this to the tube of the catheter. However, during blow moulding the wall thickness of the balloon portion thins as the balloon is expanded to the desired radial dimension. This thinning of the wall results in a fragile balloon portion and also results in excessive thinning, and hence localised extreme weakness, at the points where the fully inflated portion of the balloon merges into the narrow end portions by which the balloon is connected to the tube of the catheter. It is not practical to include re-inforcing braiding into the wall of such a blow moulded balloon, so that the weakness of the wall cannot readily be compensated for. As a result, such a construction cannot be used for balloon catheters where the diameter of the balloon is large compared to the tube to which it is to be attached. Although other methods than blow moulding could be used to form the balloon portion, these are not practical in commercial scale manufacture.
Weaknesses in the wall of the balloon portion result in a risk that the balloon will burst during inflation, notably where high inflation pressures are used. The problems due to the weaknesses in the balloon wall are accentuated when the balloon is used to expand a stent radially since the stent will typically be made from a stainless steel mesh or coil and may have sharp edges which snag the wall of the balloon. As a result, the stent readily punctures the balloon before the stent can be properly placed. It is common to use two or three balloons to place the stent. The use of replacement balloons increases the time of the procedure during which time the arterial blood flow is restricted, thus increasing patient risk and trauma, and incurring a significant additional cost.
We have now devised a form of balloon catheter which reduces the above problems.